The present invention relates to a propylene-ethylene block copolymer composition for use in automobile exterior parts such as bumper fascias, splash shields and side moldings, and more particularly to a propylene-ethylene block copolymer composition having a sufficient mechanical strength required for automobile exterior parts, which is produced at a low cost without additional steps for blending other rubber components or the like.
Hitherto, automobile exterior parts such as bumper fascias, splash shields and side moldings have been produced from polypropylene-based composite materials prepared by blending polypropylene with other components such as rubbers and talc. However, the production of these composite materials requires additional steps for blending the other components such as rubbers and talc with the polypropylene. For this reason, there has been increasing demand for developing a composition capable of being molded into automobile exterior parts by itself without blending other components, and many attempts have been made.
For example, Chikashi Okayama, xe2x80x9cChemical Economyxe2x80x9d, pp. 8-15 (October, 1996) discloses a gas-phase polymerization process for producing a polypropylene block copolymer in which the solid structure of the product is micro-controlled by carrying out the polymerization in the presence of a specific catalyst. However, the resultant composition, which contains no inorganic materials and has a specific gravity of 0.89 or less, is unsatisfactory in stiffness and mechanical strength required for automobile exterior parts since the flexural modulus is as lows as 700 MPa and the xe2x88x9230xc2x0 C. Izod impact strength is as low as 120 kJ/m2 at most.
The present invention has been made to solve the above problems. An object of the present invention is to provide a propylene-ethylene block copolymer composition for automobile exterior parts, which has sufficient stiffness and mechanical strength required for the automobile exterior parts and is produced at low cost without additional steps for blending other components such as rubbers.
As a result of extensive researches in view of the above object, the inventors have found that the object is accomplished by blending a limited amount of a nucleating agent comprising methylenebis(2,4-di-t-butylphenol) acid sodium phosphate with a propylene-ethylene block copolymer upon granulation thereof to produce a composition having specific structure and properties. The present invention has been completed based on this finding.
Namely, the propylene-ethylene block copolymer composition of the present invention is a composition for use in producing automobile exterior parts, comprising a propylene-ethylene block copolymer and a nucleating agent comprising methylenebis(2,4-di-t-butylphenol) acid sodium phosphate which is blended with the propylene-ethylene block copolymer in an amount of 300 to 2,000 ppm upon granulation thereof, and having (a) a melt flow rate (MFR) of 10 to 18 g/10 min when measured at 230xc2x0 C. under a load of 2.16 kg (21.2N); (b) an ordinary temperature xylene insoluble having a stereoregularity index [mmmm] fraction of 98.9% or higher when measured by 13C-NMR; and (c) an ordinary temperature xylene soluble characterized by: (c-1) having a content of 22 to 28% by weight; (c-2) comprising only a single component with respect to a relaxation time T1 measured by pulse NMR; and (c-3) satisfying the following formula (I):
yxe2x89xa60.0014x3xe2x88x920.0897x2xe2x88x921.0593x+231.6xe2x80x83xe2x80x83(I)
wherein x is an ethylene content (% by weight) measured by 13C-NMR and y is the relaxation time T1 (msec) measured by pulse NMR.
The propylene-ethylene block copolymer composition of the present invention has (d) a flexural modulus of 1,000 to 1,500 MPa; (e) a xe2x88x9230xc2x0 C. Izod impact strength of 6 to 8 kJ/m2; and (f) a tensile elongation of 200% or higher.
The propylene-ethylene block copolymer composition of the present invention is suitably used for the production of automobile exterior parts such as bumper fascias, splash shields and side moldings.